The present invention relates to the surface configaration of knee replacement prosthesis comprising a femoral component provided with lock pins for attachment to femoral conalyls and a plastic tibial component placed in a metal tray fixed to the tibia by means of a pin or a screw.
The injured knee joint can be replaced by an artificial knee joint. In cases where high degrees of shape deformations are to be corrected, the necessary movements can only be ensured by a tight mechanical connection of the two surfaces. Various hinged and axial total prostheses are used for such purposes. The disadvantage of the well known prostheses is that after the removal of the bone fragments the large quantity of metal is covered by only a thin layer of human tissue i.e. only by the human skin. Another disadvantage is that the bearing providing the connection can not be positioned physiologically, which may lead to the prosthesis becoming loose.
If the ligament systems providing the function of a rope system and limiting the sliding of the two surfaces on each other are intact, it is sufficient to replace only the surface participating in the movement. In these cases, so-called surface replacement prostheses are used.
In the case of these so-called surface replacement prostheses only a smaller part of the bone end must be removed. They have no mechanical connection.
The femoral component of the known surface replacement prostheses consists of two slides positioned on the condyle opposite to which a plastic block combined with a metal tray is placed on the tibia.
The femoral component consists essentially of two approximately parallel rolls as described by Fick at the beginning of the century when this has become known in the technical literature as the so-called Fick principle. The shape of these rolls is approximately ellipsoidal and the center of the generatrix curves is located on a spiral so-called evolute. This ensures a tight connection of the two bones in accordance with the components of the knee joint during flexion and extension of the knee.
The femoral component of most of the known knee prostheses is a mirrored symmetrical duplicate of prostheses corresponding to a prosthesis designed to one condyle, which, however, does not correspond to the normal threedimensional movement of the knee.
A considerable part of the known knee prostheses has a flat plane opponent element which results in a sliding of the components on each other, which damages the stability of the knee-joint. Meanwhile there is a single point connection between the curved femoral component and the plane surface causing an excessive abrasion on a plastic tibial component.
In known solutions where the opposion element is a plastic part in the shape of a trough, but due to its geometry does not allow the end rotation, therefore a rotation stress develops between the prosthesis and the bone, which may lead to a dislocation of the prosthesis component. To overcome this, rotation a rotatable plastic insert is positioned in a metal tray like rotable base. In this case the prosthesis may inhibit rotation because of the connective tissue of the human body, which causes pain. The stops limiting the movement and transmitting the forces, which may cause dislocation at the bone boundary in their end position.
Dislocations can be eliminated e.g. by application of anchoring procedures disclosed in the CH 665 55; UK 2 150 441; UK 2 219 942; U.S. Pat No. 4,659,331; U.S. Pat. No. 4,714,473 and U.S. Pat. No. 4,731,086 Patent Specifications.
In most known prostheses the depth of the trough in the sagittal plane of the opposite element is essentially constant. Such a solution is disclosed in the DE 2 550774 Patent Specification. The disadvantage of this embodiment is that the knee may be sprained backwards when the leg is extended.
The common disadvantage of the known knee prostheses discussed above is that the movements necessary during the normal human life can not be ensured without any difficulty. Meanwhile the movements or the abrasion of the components may cause a wear taking place very quickly or a mechanical loosening of the prosthesis.
The purpose of the present invention is the establishment of a prosthesis for replacing the surface of the human knee joint comprising components for realizing the normal movements necessary for perfect function of the knee-joint by sliding and rolling on each other.
During the creation of the knee prosthesis according to the invention it was started from the assumption that joints in humans and animals have evolved according to the needs of their movement and that their final shape ensures perfect functioning.
Accordingly, the essence of this invention is to form the shape of the surfaces of the prosthesis participating in the movements, which differs significantly from those of currently known knee surface replacement prostheses.
The invention is based on the recognitionxe2x80x94which has already been supported by three-dimensional analysisxe2x80x94that xe2x80x9cspherical movement of a rigid bodyxe2x80x9d known from mechanics is realized by means of the new prosthesis.
The purpose of the invention is achieved by a knee surface replacement prosthesis consisting of a femoral component provided with lock pins and a plastic tibial component placed in a metal tray fixed to the tibia by means of a pin or a screw wherein the lateral or external surface of its femoral component having a movement determined by conical surfaces, is formed by the logarithmic spiral generatrix playing a role in the movement while its medial or internal surface corresponds to the conical spiral generatrix.
In a preferred embodiment of the knee surface replacement prosthesis according to the invention the circular arcsxe2x80x94circle segmentsxe2x80x94running along the generatrix of the femoral component are shaped perpendicularly to the logarithmic spiral generatrix in the case of the lateral surface while in the case of the medial surface they are formed rotatable along the conical spiral generatrix in accordance with the segment curves of the human knee-joint.
In another preferred embodiment of the knee surface replacement prosthesis according to the invention, the surface of the tibial component is provided with a trough-shaped notch, whose surface of the exterior arch of this trough-shaped notch is formed in accordance with the rolling down the logarithmic spiral generatrix of the femoral component while of its medial surface corresponds to the rolling down of the conical spiral generatrix of the femoral component.
In a further preferred embodiment of the knee surface replacement prosthesis according to the invention the trough-shaped camber of the tibial component is higher than that of the femoral component.